The Reversible Dehydroxylization of Clay Minerals

Abstract
Rehydroxylization of the anhydrous modification of dioctahedral montmorillonites can be carried out under DTA conditions with controlled atmosphere techniques. The struc- tural character of the rehydrate prepared in this way is determined from X-ray and DTA data. The mechanism of the reactions by which the rehydrate can be formed is also considered. Thermal analysis curves of the rehydrate form of several pure mont- morillonite samples failed to show any large variation such as is shown by the curves of the original clays themselves. The possibility is considered that such large varia- tions in the dehydroxylization curves of montmorillonites are related to structural in- stead of compositional variations within the sample. It is suggested that montmoril- lonites showing two reactions attributable to dehydroxylization may be mixtures of the rehydrate form and montmorillonite. The addition of controlled-atmos phere techniques to classical differential thermal analysis (DTA) has served to stimulate new interest in its use in clay mineral research. The amount of additional information made ac- cessible with controlled atmosphere can often make the process a profitable research toot for problems in which the classical methods would fail to reveal any useful data. Not only can the usual DTA reactions be modified by heating in a controlled atmosphere, for example, analyzing specimens rich in organic material in an atmosphere of inert gas such as argon, but also new reactions can be promoted by cooling high temperature phases in an active atmosphere. Controlled-atmos phere DTA was applied in studying the reactions of clay minerals which involve crystalline or hydroxyl water. Standard DTA con- ditions of heating in an atmosphere of ordinary air at a rate of about 12~ per minute were used to produce an anhydrous form if the clay minerals. The anhydrous form was normally completely developed by 750 ~ C, as indicated by the return of the DTA curve to base line following the endothermic reaction associated with the removal of crystalline or hydroxyl water. The anhydrous material was then cooled at a controlled rate of about 10 ~ C per minute to a temperature of 400 ~ C. During the cooling an atmosphere of water vapor was maintained around the sample. The apparatus which provided these experimental conditions was similar to the one described by Stone (1952) in his thermal analysis of kaolinites, * Published with permission of the Chief, Illinois State Geological Survey, Urbana,

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